The vertical temperature and moisture profiles of the atmosphere can be determined from a space instrument containing an atmospheric sounder which views downward and measures the spectral radiance coming from the atmosphere. Art atmospheric sounder, such as the Interferometer Thermal Sounder (ITS) which was, developed during a study funded by the European Meteorological Satellite (EUMETSAT) consortium, employs a Michelson Fourier transform spectrometer to measure the spectral radiance. (EUMETSAT is a consortium of 16 countries that operates the European weather satellites and is similar to National Oceanic and Atmospheric Administration or NOAA in the United States.)
A conventional Michelson Fourier transform spectrometer (FTS) uses two mirrors, one fixed and the other moving. The FTS comprises a beamsplitter, a first plane mirror being fixed in position, and a second plane mirror which can be moved in a direction normal to the plane surface. The beamsplitter divides the incoming light into two beams, a first beam and a second beam. The first beam is reflected off the first mirror and the second beam is reflected off the second mirror. The first beam and the second beam, after reflection, recombine and interfere at the beamsplitter. The first beam, which is transmitted through the beamsplitter, and the second beam, which is reflected from the beamsplitter, are recombined to form a third beam. The third beam is processed as the second plane mirror is moved to acquire spectral information about the in-coming light.
To provide two spectral bands in a conventional FTS, it would be necessary to either (1) provide multiple interferometers, (2) share the aperture of a single interferometer, or (3) beam split the output of a single interferometer. In the first option, which involves providing multiple interferometers, the size and weight of two complete systems is a disadvantage, particularly for satellite applications. In the second option, which entails sharing the aperture of a single interferometer, efficiency is lost due to the reduced collecting area. In the third option, where the output beam of a single interferometer is split, the signal-to-noise ratio is reduced because the dwell time (to be discussed below) of the single interferometer is not optimum for both bands.
Thus, what is needed is a means to accommodate two spectral bands, each with a different spectral resolution, using a single FTS assembly such that the dwell time and hence the signal-to-noise ratio is optimized for both bands.